Generally, variable volume refrigeration units, in which a plurality of outdoor units are connected in parallel, comprises a plurality of oil balance devices connected in parallel to the compressor units.
CN Pat. No. 1707201A discloses a method for performing an oil balance operation by utilizing the compressor discharge pressure to provide the power. The lubricating oil runs subsequently to different compressors belonging to different units but the flow of lubricating oil depends on the start and stop operations of other compressors to provide a pressure difference. The cycling on and off disadvantageously influences the normal cooling and heating operation of the refrigeration units and increases the start and stop cycling of the compressors to shorten their service life. Furthermore, the balancing of the lubricating oil is driven by the power provided by the other running compressors and the balancing will have problems if one of the other compressors fails, especially when installation height gaps exist between the units.
Furthermore, in other oil balancing schemes, a complex system of pipes is required to be installed inside and outside the outdoor units and the inclination angle of the pipes are strictly limited. These restrictions lead to a high cost for the installation and the manufacture of such devices.